1. Field of the Invention
This invention relates to a catalytic reforming process wherein a suitable charge stock, such as a petroleum naphtha, is converted to a gasoline of high octane number. More particularly, the invention described herein is concerned with a startup procedure for treating a metal-containing reforming catalyst, normally sulfur-sensitive under conventional conditions of reforming operation.
2. Description of the Prior Art
Catalysts intended for use in reforming processes wherein hydrocarbon fractions, e.g., naphthas or gasoline or mixtures thereof are converted to improve the anti-knock characteristics thereof are well known in the petroleum industry.
It has heretofore been proposed to employ metal-containing catalysts, notably those containing a platinum metal, for promoting reforming. Such catalysts are necessarily characterized by a certain amount of acidity. One type of reforming catalyst which has been used commercially consists of an alumina base material having platinum metal impregnated thereon, with the acidity characteristics being contributed by a small amount of halogen incorporated in the catalyst.
In more recent years, multimetallic reforming catalysts, for example, bimetallic catalysts, have come into use. These catalysts generally contain platinum, together with one or more additional metals such as rhenium, germanium, iridium, palladium, osmium, ruthenium, rhodium, copper, silver, tin or gold deposited on a refractory support which also contains a specified amount of halogen. Representative of multimetallic reforming catalysts are those containing platinum and rhenium, such as described in U.S. Pat. No. 3,415,737; those containing platinum and iridium, such as described in U.S. Pat. Nos. 2,848,377 and 3,953,368 and those containing platinum, rhenium and iridium such as described in U.S. Pat. No. 3,487,009.
Reforming generally initially produces an excessive amount of light gases, e.g., methane and ethane, unless proper pretreatment or startup procedures are utilized. The light hydrocarbon gases, produces as a result of high hydrocracking activity or metal-cracking activity of the catalyst, are particularly to be avoided during reforming since they serve to decrease the yield of gasoline boiling products. It is known that hydrocracking activity can be diminished if the catalyst is sulfided prior to contact with the charge stock. The presulfiding can be accomplished, for example, by passing a sulfur-containing gas e.g., H.sub.2 S, through the catalyst bed. Other presulfiding treatments utilizing various other sulfur-containing compounds are known from prior art, such as U.S. Pat. No. 3,415,737.
While generally any of the aforenoted metal-containing reforming catalysts are adversely affected by the presence of an excess amount of sulfur, i.e., greater than about 15 ppm, those in which iridium is a catalytically active component are known to be extremely sensitive to the presence of sulfur. Thus, it has been reported, for example, in U.S. Pat. No. 3,507,781, that reforming catalysts comprising catalytically active amounts of platinum and iridium supported on a porous solid carrier, for example, alumina, are extremely sensitive to sulfur concentrations, exceeding about 2 ppm. At such concentrations, the increase in catalyst temperature necessary to maintain conversion of the chargestock to a constant octane number gasoline product increases very substantially.
During the startup period of a reforming unit, utilizing a metal, e.g., a platinum-iridium-containing catalyst, that is, when the catalyst is initially or immediately after regeneration contacted with hydrogen and naphtha at reforming conditions, the catalyst causes excessive hydrocracking which has been termed "hydrogenolysis". As a consequence of such high hydrocracking activity, an excessive temperature rise or heat front, travels through the catalyst as naphtha is initially contacted with the catalyst in the presence of hydrogen and at reforming conditions. Although the occurring temperature rise only exists in the initial period of contact with the naphtha feed, such could be the cause of a temperature runaway in a commercial reforming plant. The temperatures in the bed may increase as high as several hundred degrees above the temperature of the naphtha introduced to the reaction zone. Obviously, such a severe temperature increase can damage the reactor and/or catalyst and is to be strictly avoided.
One method of controlling the hydrocracking activity of the platinum-containing reforming catalyst, e.g., platinum in combination with iridium and/or rhenium catalyst, would be to add a quantity of sulfur to the feed during the startup period. However, such catalyst, as indicated above, is very sensitive to the presence of sulfur and other means of control have accordingly been sought.
One alternative suggested method is that described in U.S. Pat. No. 3,507,781 wherein a reforming process using a catalyst containing platinum and iridium on a porous solid carrier is started up by contacting the naphtha with the catalyst in the presence of an inert gas, for example, nitrogen. Utilizing such technique, it has been indicated that the pressure in the reforming zone should be about 200 psig and the catalyst temperature about 650.degree. F. when the naphtha is first contacted with the catalyst at a space velocity of about 1 volume/volume/hour. Thereafter, the temperature is increased to about 900.degree. F. over a 2-3 hour period while building up autogeneous pressure of produced hydrogen.
Another method is that described in U.S. Pat. No. 4,148,758 wherein excessive hydrocracking or hydrogenolysis of a sulfur sensitive reforming catalyst is suppressed by incorporating within the reforming catalyst at the time of its preparation a sulfurous acid or sulfuric acid component.
Such prior suggested alternative techniques have had the disadvantage of requiring extremely careful control of treating conditions or with respect to the method described in the latter patent the use of corrosive chemicals.